My GSD, Baron, was the best dog I've ever known. He was the most loyal and protective animal to me and my family. He had the same issues as your dog and couldn't move his back legs anymore. I was putting that day with the vet off for a little more time with him.
9/11 happened and forced me to make that decision a little sooner than I wanted cause we were being deployed that week.
I still miss him.
So sorry for your loss.
